Sky & Telescope - September 2021 - 46

SEPTEMBER 2021 OBSERVING
Sun, Moon & Planets by Gary Seronik
To find out what's
visible in the sky from
your location, go to
skyandtelescope.org.
Low-lying Planets Greet the Moon
Both Mercury and Venus are visible at dusk - one is easy, one is not.
SATURDAY, SEPTEMBER 4
Broadly speaking, Mercury apparitions
come in two flavors: short and high,
and long and low. Mercury's current
showing is of the latter variety - it's
simultaneously the longest of the year
and presents the planet's poorest visibility.
And because the planet remains
mired in horizon muck during its longest
apparitions, Mercury has a reputation
for being difficult to see.
The innermost planet doesn't reach
its greatest elongation from the Sun
until the 14th. However, thanks to the
shallow angle the ecliptic makes to the
western horizon at dusk at this time
of year, Mercury attains its greatest
altitude for observers at mid-northern
latitudes 10 days early. Even using the
phrase " greatest altitude " here misrepresents
the situation. At the end of
civil twilight, the planet stands just a
hair more than 4° above the western
horizon (as it does from the 2nd to the
6th, inclusive). Even though Mercury
shines at magnitude +0.0, its low
altitude makes it a tough find. I expect
you'll require binoculars to have any
hope of sighting the speedy planet this
time around, and a telescope equipped
with a Go To mount would be an even
better choice. Or you could simply wait.
It'll be much easier to see Mercury at
dawn next month when it has one of
those " short and high " apparitions I
mentioned earlier.
SUNDAY, SEPTEMBER 5
Like Mercury, Venus is also in the
midst of a lengthy dusk apparition.
The brilliant world has reigned as the
Evening Star since it emerged from
its conjunction with the Sun back in
March, and it will continue to adorn
the evening sky well into January 2022.
Venus currently shines at a magnificent
magnitude -4.1 and is both much
brighter and much higher than Mercury.
In fact, Venus is hard to miss.
What you might overlook with a casual
glance, however, is Spica shining nearby.
Venus has been closing in on Spica for
some time, and this evening it's at its
very closest, with just 1½° separating
it from the 1st-magnitude star. If you
want to catch the pairing when it's at
its highest, seek out Venus soon after
sunset. Once you've located the planet,
use your binoculars to find Spica below
and left. As twilight fades, it'll be easier
to spot Spica, and eventually you'll be
able to dispense with the optical aid.
Mercury has its own encounter with
Spica on the 21st, but that particular
event will be very difficult to observe
since both objects will be extremely low
These scenes are drawn for near the middle of North America (latitude 40° north, longitude 90° west). European observers should move each
Moon symbol a quarter of the way toward the one for the previous date; in the Far East, move the Moon halfway. The blue 10° scale bar is about the
width of your fist at arm's length. For clarity, the Moon is shown three times its actual apparent size.
Dusk, Sept 5
30 minutes after sunset
VIRGO
VIRGO
10°
Venus
Venus
Spica
Mercury
γ
Spica
Moon
Sept 8
γ
Mercury
Looking West-Southwest
46
SEPTEMBER 2021 * SKY & TELESCOPE
Looking West-Southwest
Looking South-Southwest
Moon
Sept 9
SCORPIUS
Dusk, Sept 8 - 9
30 minutes after sunset
Dusk, Sept 12 -13
1 hour after sunset
Moon
Sept 13
Antares
Moon
Sept 12

http://www.skyandtelescope.org

Sky & Telescope - September 2021

Table of Contents for the Digital Edition of Sky & Telescope - September 2021